Sodium-rich sodium iron pyrophosphate cathode material, preparation method and application

Abstract

The sodium-rich sodium iron pyrophosphate phosphate cathode material disclosed by the present invention, its preparation method and application, with the molecular formula of Na 4+x Fe 3‑y M y (PO4)2P2O7F x @C; in the molecular formula, 0 < x ≤ 0.1, 0 ≤ y < 3, and M is one or a mixture of two or more of Mn, Co, Ni, Ti, Mg, and Al. For the sodium-rich sodium iron pyrophosphate phosphate cathode material disclosed by the present invention, its preparation method and application, after synthesizing the precursor of the sodium-rich sodium iron pyrophosphate phosphate cathode material by the ball milling method, the finally required sodium-rich sodium iron pyrophosphate phosphate cathode material is obtained by heat treatment, providing an idea for the development of new cathode materials for sodium-ion batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of new cathode materials for sodium-ion batteries, and particularly relates to a sodium-rich sodium iron pyrophosphate phosphate cathode material. The present invention also relates to a preparation method of the sodium-rich sodium iron pyrophosphate phosphate cathode material. The present invention further relates to a method for using the sodium-rich sodium iron pyrophosphate phosphate cathode material to prepare a cathode plate of a sodium-ion battery. Background Art

[0002] With the continuous deterioration of global environmental problems and the intensification of energy crises, there is an urgent need for clean and renewable energy. In recent years, market demand has continuously promoted the development of the energy storage field towards electrochemical energy storage. Compared with traditional cathode materials for lithium-ion batteries, such as lithium cobaltate, lithium manganate, ternary materials, etc., the sodium-ion battery cathode material sodium iron pyrophosphate phosphate (Na4Fe3(PO4)2P2O7, denoted as NFPP) has the advantages of rich reserves, wide distribution, low price, and stable structure. This characteristic exactly matches the characteristics of large-scale energy storage devices. Therefore, sodium-ion batteries are considered to be one of the potential candidates for large-scale energy storage systems. For NFPP, it has a relatively high theoretical capacity (~129 mAh / g), a relatively high working voltage (~3.1 V, Na + / Na) and a relatively low volume expansion (~4%), and is considered to be the most promising cathode material for sodium-ion batteries. Among them, the applicant's research found that by adding an excessive amount of sodium ions to the material and introducing dual doping of cations and anions, the discharge specific capacity of the material can be effectively improved, and the cycle stability of the material can be improved. This discovery provides an idea for improving the material performance. Summary of the Invention

[0003] The purpose of the present invention is to provide a sodium-rich sodium iron pyrophosphate phosphate cathode material, which solves the problem that the existing electrochemical energy storage overly relies on cathode materials for lithium-ion batteries.

[0004] Another purpose of the present invention is to provide a preparation method of a sodium-rich sodium iron pyrophosphate phosphate cathode material.

[0005] Another purpose of the present invention is to provide a method for using a sodium-rich sodium iron pyrophosphate phosphate cathode material to prepare a cathode plate of a sodium-ion battery.

[0006] The first technical solution adopted by the present invention is: a sodium-rich sodium iron pyrophosphate phosphate cathode material, with the molecular formula of Na 4+x Fe 3-y M y (PO4)2P2O7F x @C; in the molecular formula, 0 < x ≤ 0.1, 0 ≤ y < 3, and M is one or a mixture of two or more of Mn, Co, Ni, Ti, Mg, and Al.

[0007] The first technical solution of the present invention is further characterized in that

[0008] The positive electrode material is particles with an average particle size of 50 to 500 nm, and the average particle size is preferably 100 to 300 nm.

[0009] The second technical solution adopted by the present invention is: a preparation method of a sodium-rich sodium iron pyrophosphate positive electrode material, comprising the following steps:

[0010] Step 1: Put pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt into a ball milling tank;

[0011] Step 2: Add absolute ethanol to the ball milling tank;

[0012] Step 3: Start the ball mill and perform high-speed ball milling;

[0013] Step 4: After drying the product obtained in Step 3, a powdery mixed precursor is obtained;

[0014] Step 5: Then heat-treat the precursor obtained in Step 4 with a carbon source in an inert reducing atmosphere to obtain the product.

[0015] The second technical solution of the present invention is further characterized in that

[0016] The pyrophosphate in Step 1 is one or a mixture of two of sodium pyrophosphate and potassium pyrophosphate, preferably sodium pyrophosphate; the phosphate is one or a mixture of two or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid, preferably ammonium dihydrogen phosphate; the iron salt is one or a mixture of two or more of ferrous acetate, iron nitrate, ferrous oxalate, and reduced iron powder, preferably ferrous oxalate; the sodium salt is one or a mixture of two or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium citrate, preferably sodium carbonate; the fluoride is one or two of sodium fluoride and ammonium fluoride; the doping salt is one or a mixture of two or more of oxides or carbides of Mn, Co, Ni, Ti, Mg, and Al, preferably one or a mixture of two or more of manganese oxide, cobalt oxide, nickel oxide, and titanium oxide.

[0017] The mixing ratio of pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt in Step 1 is a molar ratio of 1:2:0 to 3:4 to 4.1:0 to 0.1:0 to 3, preferably 1:2:2.99:4.01:0.01:0.01.

[0018] The amount of absolute ethanol used in Step 2 is determined according to the total amount of salts of pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt in Step 1, and the mixing ratio of absolute ethanol and the total amount of salts is calculated according to a mass ratio of 0 to 2:1, preferably 0.5:1.

[0019] In Step 3, the rotation speed of the high-speed ball milling is 300 - 500 rpm, preferably 400 rpm; the ball milling time is 10 - 30 h, preferably 24 h.

[0020] In Step 4, the drying method is one of spray drying, blow drying, vacuum drying, and freeze drying.

[0021] In Step 5, the inert reducing atmosphere is one or a mixture of two or more of nitrogen, argon, and hydrogen, preferably a mixture of argon and hydrogen with a hydrogen volume percentage of 5%; the carbon source is one or a mixture of two or more of starch, citric acid, sucrose, and glucose, preferably citric acid; the carbon source accounts for 5 - 15 wt.% in the whole of it and the precursor, preferably 10 wt.%; the heat treatment is divided into two steps: pre-calcination and high-temperature calcination: Pre-calcination process: the heating rate is 1 - 5 °C / min, preferably 2 °C / min; the temperature is 250 - 350 °C, preferably 300 °C; the holding time is 3 - 10 h, preferably 6 h; the cooling is furnace cooling; High-temperature calcination process: the heating rate is 1 - 5 °C / min, preferably 2 °C / min; the temperature is 450 - 550 °C, preferably 500 °C; the holding time is 5 - 15 h, preferably 10 h; the cooling is furnace cooling.

[0022] The third technical solution adopted in the present invention is: A method for preparing a positive electrode sheet of a sodium-ion battery using a sodium-rich sodium pyrophosphate iron phosphate positive electrode material, comprising the following steps:

[0023] Step 1: Weigh the following components by mass parts: 80 parts of the above-mentioned sodium-rich sodium pyrophosphate iron phosphate positive electrode material, 10 parts of acetylene black, and 10 parts of binder;

[0024] Step 2: Dissolve the sodium-rich sodium pyrophosphate iron phosphate positive electrode material, acetylene black, and binder weighed in Step 1 in N-methylpyrrolidone, then coat it on the treated aluminum foil and dry it to obtain.

[0025] The beneficial effects of the present invention are: For the sodium-rich sodium pyrophosphate iron phosphate positive electrode material, its preparation method and application of the present invention, after synthesizing the precursor of the sodium-rich sodium pyrophosphate iron phosphate positive electrode material by the ball milling method, the final required sodium-rich sodium pyrophosphate iron phosphate positive electrode material is obtained by heat treatment, providing an idea for the development of new positive electrode materials for sodium-ion batteries. Description of the Drawings

[0026] Figure 1 is the electron microscope image of the sodium-rich sodium pyrophosphate iron phosphate positive electrode material of the present invention;

[0027] Figure 2 is the XRD pattern of the sodium-rich sodium pyrophosphate iron phosphate positive electrode material prepared in Example 2 of the present invention;

[0028] Figure 3It is the charge-discharge curve of the button battery in Example 4 of the present invention at a rate of 0.1C;

[0029] Figure 4 It is the cycle curve of the button battery in Example 4 of the present invention at a rate of 1C. Detailed implementation manners

[0030] The present invention will be described in detail below with reference to the accompanying drawings and specific implementation manners.

[0031] The present invention provides a sodium-rich sodium iron pyrophosphate phosphate cathode material, as Figure 1 shown, with the molecular formula Na 4+ x Fe 3-y M y (PO4)2P2O7F x @C, where 0 < x ≤ 0.1, 0 ≤ y < 3, and M is one or more of Mn, Co, Ni, Ti, Mg, and Al. The cathode material is particles with an average particle size of 50 to 500 nm, preferably 100 to 300 nm.

[0032] The present invention also provides a preparation method for the above sodium-rich sodium iron pyrophosphate phosphate cathode material, including the following steps:

[0033] Step 1: The mixing ratio of pyrophosphate, phosphate, iron salt, sodium salt, fluoride, and doping salt is a molar ratio of 1:2:0 to 3:4 to 4.1:0 to 0.1:0 to 3, preferably 1:2:2.99:4.01:0.01:0.01. Among them, the pyrophosphate is one or a mixture of two of sodium pyrophosphate and potassium pyrophosphate, preferably sodium pyrophosphate; the phosphate is one or a mixture of two or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid, preferably ammonium dihydrogen phosphate; the iron salt is one or a mixture of two or more of ferrous acetate, iron nitrate, ferrous oxalate, and reduced iron powder, preferably ferrous oxalate; the sodium salt is one or a mixture of two or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium citrate, preferably sodium carbonate; the fluoride is one or a mixture of two of sodium fluoride and ammonium fluoride; the other doping salts are one or a mixture of two or more of Mn, Co, Ni, Ti, Mg, and Al related oxides and carbides, preferably a mixture of two or more of oxides, and more preferably a mixture of two or more of manganese oxide, titanium oxide, nickel oxide, and cobalt oxide.

[0034] Step 2: Add absolute ethanol to the ball mill. The amount of absolute ethanol is determined according to the total amount of pyrophosphate, phosphate, iron salt, sodium salt, fluoride, and doping salt in Step 1. The mixing ratio of absolute ethanol and the total amount of the above salts is calculated according to a mass ratio of 0 to 2:1, preferably 0.5:1.

[0035] Step 3: Start the ball mill and perform high-speed ball milling. Among them, the high-speed ball milling speed is 300 - 450 rpm, preferably 400 rpm; the ball milling time is 10 - 30 h, preferably 24 h.

[0036] Step 4: After drying the product obtained in Step 3, a powdery mixed precursor is obtained. Among them, the drying method is one of spray drying, air drying, vacuum drying, and freeze drying.

[0037] Step 5: Then, heat-treat the precursor obtained in Step 4 and the carbon source under an inert atmosphere to obtain the product. Among them, the inert atmosphere is one or a mixture of two or more of nitrogen, argon, and hydrogen, preferably a mixture of argon and hydrogen, and the volume percentage of hydrogen is 5%. The carbon source is one or a mixture of two or more of starch, citric acid, sucrose, and glucose, preferably citric acid; the proportion of the carbon source in the material is 5 - 15 wt.%, preferably 10%. The heat treatment is divided into two steps: pre-burning and high-temperature calcination. Pre-burning process: the heating rate is 1 - 5 °C / min, preferably 2 °C / min; the temperature is 250 - 350 °C, preferably 300 °C; the holding time is 3 - 10 h, preferably 6 h; the cooling is by furnace cooling. High-temperature calcination process: the heating rate is 1 - 5 °C / min, preferably 2 °C / min; the temperature is 450 - 550 °C, preferably 500 °C; the holding time is 5 - 15 h, preferably 10 h; the cooling is by furnace cooling.

[0038] The present invention also provides a method for preparing a positive electrode sheet of a sodium-ion battery using the above-mentioned sodium-rich sodium iron pyrophosphate positive electrode material, including the following steps:

[0039] Step 1: Weigh the following components by mass parts: 80 parts of sodium-rich sodium iron pyrophosphate positive electrode material, 10 parts of acetylene black, and 10 parts of binder;

[0040] Step 2: Dissolve the sodium-rich sodium iron pyrophosphate positive electrode material, acetylene black, and binder weighed in Step 1 in N-methylpyrrolidone, then coat it on the treated aluminum foil and dry it to obtain a sodium-rich sodium iron pyrophosphate positive electrode sheet.

[0041] After that, assemble it with a sodium sheet into a coin cell in a glove box for subsequent electrochemical performance testing. The obtained sodium-rich sodium iron pyrophosphate electrode sheet is the positive electrode, the sodium sheet is the counter negative electrode, and the electrolyte is 1 molL -1 A sodium salt of NaClO4 is dissolved in a solution of ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1, and an additive of 5% fluoroethylene carbonate (FEC) is additionally added to the electrolyte. During the electrochemical performance testing, the current of 1C is 129 mA / g, and the charge and discharge temperature is room temperature.

[0042] Example 1

[0043] Weigh sodium pyrophosphate, ferrous oxalate dihydrate, nickel oxide, cobalt oxide, titanium oxide, ammonium dihydrogen phosphate, and sodium fluoride according to a molar ratio of 1:0.1:2:0.5:0.4:2:0.1, where 256.75 mg of sodium pyrophosphate is used as a reference. Add 2 ml of absolute ethanol dropwise and ball-mill for 30 hours at a rotation speed of 300 rpm. After spray drying, a precursor powder is obtained. Grind the precursor powder and 5 wt.% of the carbon source citric acid, then heat it in a tubular vacuum furnace to 250 °C and hold for 10 hours, and then heat to 450 °C and hold for 15 hours. Protect it with a mixed gas of argon and hydrogen throughout the process, with a heating rate of 1 °C / min. After cooling with the furnace, the required Na 4.1 Fe 0.1 Ni2Co 0.5 Ti 0.4 (PO4)2P2O7F 0.1 @C cathode material.

[0044] Example 2

[0045] Weigh sodium pyrophosphate, ferrous oxalate dihydrate, manganese oxide, ammonium dihydrogen phosphate, and sodium fluoride according to a molar ratio of 1:2.99:0.01:2:0.01, where 256.75 mg of sodium pyrophosphate is used as a reference. Add 2 ml of absolute ethanol dropwise and ball-mill for 24 hours at a rotation speed of 400 rpm. After vacuum drying, a precursor powder is obtained. Grind the precursor powder and 10 wt.% of the carbon source citric acid, then heat it in a tubular vacuum furnace to 300 °C and hold for 6 hours, and then heat to 500 °C and hold for 10 hours. Protect it with a mixed gas of argon and hydrogen throughout the process, with a heating rate of 2 °C / min. After cooling with the furnace, the required Na 4.01 Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 @C cathode material.

[0046] Example 3

[0047] Weigh sodium pyrophosphate, ferrous oxalate dihydrate, magnesium oxide, aluminum oxide, manganese oxide, ammonium dihydrogen phosphate, and sodium fluoride according to a molar ratio of 1:2.94:0.02:0.02:0.02:2:0.01, where 256.75 mg of sodium pyrophosphate is used as a reference. Add 2 ml of absolute ethanol dropwise and ball-mill for 10 hours at a rotation speed of 450 rpm. After freeze-drying, a precursor powder is obtained. Grind the precursor powder and 15 wt.% of the carbon source citric acid, then heat it in a tubular vacuum furnace to 350 °C and hold for 3 hours, and then heat to 550 °C and hold for 5 hours. Protect it with a mixed gas of argon and hydrogen throughout the process, with a heating rate of 5 °C / min. After cooling with the furnace, the required Na 4.01 Fe 2.94 Mg 0.02 Al 0.02 Mn0.02 (PO4)2P2O7F 0.01 @C cathode material.

[0048] Example 4

[0049] Take 0.4 g of the cathode material Na 4.01 Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 @C, 0.05 g of conductive agent (acetylene black), 0.05 g of binder (polyvinylidene fluoride (PVDF)), and mix them in 0.5 g of N-methylpyrrolidone (NMP) to obtain the required cathode slurry. Subsequently, coat the above slurry on the treated aluminum foil and dry it in a vacuum drying oven at 90 °C for 10 h to obtain the cathode electrode sheet. Assemble the above electrode sheet and sodium sheet into a button cell in a glove box. Among them, the electrolyte is 1 molL -1 The sodium salt of NaClO4 is dissolved in a solution of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) with a volume ratio of 1:1, and 5% fluoroethylene carbonate (FEC) additive is additionally added to the electrolyte. The subsequent electrochemical performance tests will mainly focus on the button cell.

[0050] Test Example 1

[0051] Perform X-ray diffraction (SHIMADZU XRD-7000) tests on the Na 4.01 Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 @C cathode material prepared in Example 2. The experimental conditions are as follows: copper target (λ = 0.1518 nm), and the 2θ angle range is 5 - 70°. The XRD pattern is as Figure 2 shown.

[0052] From Figure 2 the XRD pattern in, it can be seen that Na 4.01 Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 @C cathode material has a pure-phase crystal structure with a high crystallinity Pn21a space group (PDF standard card number: PDF#89-0579). No obvious impurity diffraction peaks are observed, indicating that adding a small amount of surplus sodium ions, doping part of manganese and fluorine elements does not affect the crystal structure of this material. This result shows that the experimental method in this experiment can prepare a pure-phase and high-crystallinity sodium-rich sodium iron pyrophosphate cathode material.

[0053] Test Example 2

[0054] Perform the Na 4.01Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 The @C cathode material was observed by scanning electron microscopy (FESEM, JSM-6700F). The specific images are as Figure 1 shown.

[0055] As Figure 1 shown, Na 4.01 Fe 2.99 Mn 0.01 (PO4)2P2O7F 0.01 The @C cathode material as a whole shows irregular particles with particle sizes between 50 and 500 nm. Among them, due to the presence of the carbon source, a small amount of aggregation occurs in the primary particles, and the overall size of the aggregated secondary particles is less than 3 microns. This indicates that the experimental process parameters are relatively appropriate and can synthesize a sodium-rich sodium iron pyrophosphate cathode material with small size and concentrated distribution.

[0056] Test Example 3

[0057] The button cell prepared in Example 4 was subjected to charge-discharge tests. The current of 1C is 129 mA / g, and the charge-discharge temperature is room temperature.

[0058] Figure 3 The charge-discharge curves of the button cell assembled in Example 4 are shown. The charge-discharge current is 0.1C, and the charge-discharge voltage range is 1.8 - 3.8V. In the figure, the curve marked 1 is the charge curve, and the curve marked 2 is the discharge curve. It can be seen from the figure that during the entire charge-discharge process, the charge-discharge curves are smooth and complete, indicating that the battery can be charged and discharged well; the charge specific capacity is 119 mAh / g, the discharge specific capacity is 108 mAh / g, and the first-cycle Coulomb efficiency is 90.7%. The above data show that this sodium-rich sodium iron pyrophosphate cathode material has a relatively high discharge specific capacity.

[0059] Test Example 4

[0060] The button cell prepared in Example 4 was subjected to charge-discharge tests. The current of 1C is 129 mA / g, and the charge-discharge temperature is room temperature.

[0061] Figure 4 The 1C cycle curves of the button cell assembled in Example 4 are shown. The charge-discharge voltage range is 1.8 - 3.8V. It can be seen from the figure that the initial discharge specific capacity during cycling is 86 mAh / g. After 200 cycles, the discharge specific capacity is 80 mAh / g, and the capacity retention rate is 93%. The above data show that this sodium-rich sodium iron pyrophosphate cathode material has relatively high cycle stability.

[0062] By the above method, for the sodium-rich sodium iron pyrophosphate cathode material of the present invention, stoichiometric pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt are mixed and ball-milled by the ball-milling method, and the obtained precursor is heat-treated under a reducing atmosphere to finally obtain the sodium-rich sodium iron pyrophosphate cathode material. The obtained nanomaterial has good electrochemical behavior and is used as the cathode of a rechargeable sodium-ion battery. In the process of material preparation of the present invention, an excessive amount of sodium ions is added, and at the same time, dual doping of anions and cations is introduced, which can effectively improve the discharge specific capacity of the material and improve the cycle stability of the material.

Claims

1. Sodium-rich sodium iron pyrophosphate cathode material, characterized in that, The molecular formula is Na 4+x Fe 3-y M y (PO4)2P2O7F x @C; in the molecular formula, 0 < x ≤ 0.1, 0 ≤ y < 3, and M is one or a mixture of two or more of Mn, Co, Ni, Ti, Mg, and Al.

2. The sodium-rich sodium iron pyrophosphate cathode material according to claim 1, wherein, The cathode material is particles with an average particle size of 50 to 500 nm.

3. The sodium-rich sodium iron pyrophosphate cathode material according to claim 1, wherein The cathode material is particles with an average particle size of 100 to 300 nm.

4. Preparation method of sodium-rich sodium iron pyrophosphate cathode material, characterized in that, It includes the following steps: Step 1: Put pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt into a ball milling tank; the pyrophosphate is one or a mixture of two of sodium pyrophosphate and potassium pyrophosphate; the phosphate is one or a mixture of two or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid; the iron salt is one or a mixture of two or more of ferrous acetate, iron nitrate, ferrous oxalate, and reduced iron powder; the sodium salt is one or a mixture of two or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium citrate; the fluoride is one or a mixture of two of sodium fluoride and ammonium fluoride; the doping salt is one or a mixture of two or more of oxides or carbides of Mn, Co, Ni, Ti, Mg, and Al; the mixing ratio of pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt is a molar ratio of 1:2:0 to 3:4 to 4.1:0 to 0.1:0 to 3; Step 2: Add anhydrous ethanol to the ball milling tank. The amount of anhydrous ethanol is determined according to the total amount of salts of pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt in Step 1. The mixing ratio of anhydrous ethanol and the total amount of salts is calculated according to a mass ratio of 0 to 2:1; Step 3: Start the ball mill and perform high-speed ball milling. The rotation speed of the high-speed ball milling is 300 to 500 rpm, and the ball milling time is 10 to 30 h; Step 4: After drying the product obtained in Step 3, a powdery mixed precursor is obtained; Step 5: Then heat-treat the precursor obtained in Step 4 with a carbon source under an inert reducing atmosphere to obtain it; wherein the inert reducing atmosphere is one or a mixture of two or more of nitrogen, argon, and hydrogen, and the volume percentage of hydrogen is 5%; the carbon source is one or a mixture of two or more of starch, citric acid, sucrose, and glucose, and the carbon source accounts for 5 to 15 wt.% in the whole of it and the precursor; the heat treatment is divided into two steps of pre-burning and high-temperature calcination: Pre-burning process: the heating rate is 1 to 5 °C / min, the temperature is 250 to 350 °C, the holding time is 3 to 10 h, and the cooling is furnace cooling; High-temperature calcination process: the heating rate is 1 to 5 °C / min, the temperature is 450 to 550 °C, the holding time is 5 to 15 h, and the cooling is furnace cooling.

5. The preparation method of the sodium-rich sodium iron pyrophosphate phosphate cathode material according to claim 4, wherein In Step 1, the pyrophosphate is sodium pyrophosphate, the phosphate is ammonium dihydrogen phosphate, the iron salt is ferrous oxalate, the sodium salt is sodium carbonate, and the doping salt is one or a mixture of two or more of manganese oxide, cobalt oxide, nickel oxide, and titanium oxide.

6. The preparation method of the sodium-rich sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In Step 1, the mixing ratio of pyrophosphate, phosphate, iron salt, sodium salt, fluoride and doping salt is a molar ratio of 1:2:2.99:4.01:0.01:0.

01.

7. The preparation method of the sodium-rich sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In Step 2, the mixing ratio of anhydrous ethanol and the total amount of salts is calculated according to a mass ratio of 0.5:

1.

8. The preparation method of the sodium-rich sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In Step 3, the rotation speed of the high-speed ball milling is 400 rpm; the ball milling time is 24 h.

9. The preparation method of the sodium-rich sodium iron pyrophosphate cathode material according to claim 4, characterized in that, In Step 4, the drying method is one of spray drying, air drying, vacuum drying, and freeze drying.

10. The preparation method of the sodium-rich sodium iron pyrophosphate positive electrode material according to claim 4, characterized in that, In step 5, the inert reducing atmosphere is a mixed gas of argon and hydrogen, and the volume percentage of hydrogen is 5%; the carbon source is citric acid, and the carbon source accounts for 10 wt.% in the whole of it and the precursor.

11. The preparation method of the sodium-rich sodium iron pyrophosphate phosphate cathode material according to claim 4, characterized in that, In step 5, the heat treatment is divided into two steps: pre-calcination and high-temperature calcination: Pre-calcination process: the heating rate is 2 °C / min, the temperature is 300 °C, the heat preservation time is 6 h, and the cooling is carried out by furnace cooling; High-temperature calcination process: the heating rate is 2 °C / min, the temperature is 500 °C, the heat preservation time is 10 h, and the cooling is carried out by furnace cooling.

12. Method for preparing positive electrode sheet of sodium ion battery using sodium-rich sodium iron pyrophosphate as positive electrode material, characterized in that, It includes the following steps: Step 1: Weigh the following components by mass parts: 80 parts of the sodium-rich sodium pyrophosphate iron phosphate cathode material as described in claim 1, 10 parts of acetylene black, and 10 parts of binder; Step 2: Dissolve the sodium-rich sodium pyrophosphate iron phosphate cathode material, acetylene black, and binder weighed in step 1 in N-methylpyrrolidone, then coat it on the treated aluminum foil and dry it to obtain.

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